Average API (American Petroleum Institute) gravity of conventional crude oil is decreasing due to presence of more residual content that boils above 550° C. and contains higher levels of asphaltenes, resins, concarbon and metal components. Use of high-boiling residue as heavy fuel oil or furnace oil is continuously decreasing due to stringent environmental regulations. This places greater emphasis on Refiners to process the entire barrel of crude oil to produce more valuable lower boiling products. Residue of high sulfur crude oil from vacuum distillation column bottom contains about 35-65% resins and asphaltenes, 20-30 wt % concarbon and 50-200 ppm metals. Concarbon & metals are mostly concentrated in the last 10% of crude oil. As vacuum residue contains larger amount of impurities, which are poisons to the catalyst, hence is not a preferred feedstock for catalytic cracking units. Various thermal cracking methods such as visbreaking, coking, etc. have been used to upgrade residue feedstock.
Under condition of thermal cracking, hydrocarbons when heated, long chain complex molecules crack into smaller hydrocarbon molecules. Asphaltene in the feed remain unaffected and additional asphaltenes may be formed via secondary polymerization reactions. Beyond certain temperature & residence time, asphaltenes stability get disturbed and precipitated due to cracking of resins and aromatic compounds. At this conversion, the product residue becomes unstable. Hence, visbreaking is conducted at mild operating conditions so that the stability of asphaltene is not disturbed. In visbreaking process, concarbon and metals are not removed from the visbroken product. As the product is typically used for producing fuel oil and demand for fuel oil is declining, the process is not widely used in the industry.
U.S. Pat. No. 6,540,904 discloses a process for upgradation of petroleum residue using Fe based catalyst along with almost 50% of water. However, the patent does not discuss the stability of the product.
U.S. Pat. No. 4,615,791 discloses a process for carrying out visbreaking operation at higher severity using hydrogen donor solvent for reducing the coke formation and producing a product of reduced viscosity, pour point and sedimentation characteristics.
U.S. Pat. No. 5,057,204 describes a process for increasing severity in visbreaking process using SeO2 as a catalyst, which helps in promoting transfer of hydrogen from residue feed to the portion of the feed having reactive radicals formed during the reaction. This patent does not disclose the use of hydrogen and aromatic rich material, which helps in increasing visbreaking unit severity by enhancing solvency power of the hydrocarbon oil for keeping asphaltenes in dispersed phase.
U.S. Pat. No. 6,193,875 discloses a method for making an oil soluble coking process additive, including the steps like providing mixture of a metal salt in water wherein the metal salt contains a metal selected from the group consisting of alkali metals, alkaline earth metals and mixtures thereof; and the other step includes providing a heavy hydrocarbon and forming a emulsion so as to react a metal salt with components of the heavy hydrocarbon so as to provide a treated hydrocarbon containing oil soluble organometallic compounds.
Coking is very severe form of thermal cracking and converts the heaviest low value residue to valuable distillates and petroleum coke. Two types of coking processes most commonly & commercially practiced are delayed coking and fluidized bed coking. In delayed coking, the residue is heated in a furnace and passed to large drums maintained at temperatures from about 450-500° C. During the reaction, the colloidal suspension of the asphaltenes and resin compounds is distorted, resulting in precipitation of highly cross linked structure of amorphous coke. The compounds are also subjected to cleavage of the aliphatic groups. Polymerization and condensation of the free aromatic radicals and grouping of a large number of these compounds to such a degree that significant amount of coke is eventually formed along with some lighter gas and low boiling liquid products.
Conventional fluidized bed coking process units typically include a coking reactor and a burner. Residue feedstock is introduced into the coking reactor containing a fluidized bed of hot inert coke particles, and is distributed uniformly over the surfaces of the particles, where it is cracked into vapours and coke. A fraction of coke is burnt in burner to supply the endothermic heat required during coking process. The remaining coke is drawn off the burner vessel. Alternatively, in flexi-coking process, the coke is gasified with air & steam in a separate vessel.
Coking processes produces lower liquid yield & high amount of low value petroleum coke, typically 1.5 times of concarbon, which means, processing of heavy crude oil having 40% VR with 27% concarbon will produce 16 MT of coke as by product per every 100 MT of crude oil. The price of coke is very low as compared to crude oil price, approximately 1/10th of crude oil price and it erodes the refinery margin heavily. While residue can be upgraded in petroleum refineries via coking process, there is still a substantial need to have an efficient process for producing more liquid yield and to decrease the amount of gas and/or coke make, when upgrading such feedstocks.